This relates generally to imaging devices, and more particularly, to imaging devices with photodiodes having charge-summing capabilities.
Image sensors are commonly used in electronic devices such as cellular telephones, cameras, and computers to capture images. In a typical arrangement, an electronic device is provided with an array of image pixels arranged in pixel rows and pixel columns. The image pixels contain a single photodiode for generating charge in response to image light. The charge generated by the image pixels is stored at a floating diffusion node coupled to the image pixels. Circuitry is commonly coupled to each pixel column for reading out image signals from the image pixels.
Conventional imaging systems employ a single image sensor in which the visible light spectrum is sampled by red, green, and blue (RGB) image pixels arranged in a Bayer mosaic pattern. The Bayer mosaic pattern consists of a repeating unit cell of two-by-two image pixels, with two green pixels diagonally opposite one another, and the other corners being red and blue.
In certain applications, it may be desirable to store charge from multiple pixels on a single shared floating diffusion node. In conventional imaging systems that implement a Bayer mosaic pattern, adjacent pixels of different colors in a single column of the image pixel array or adjacent pixels in a Bayer mosaic unit cell share a common floating diffusion node on which charge from the pixels is be stored prior to read-out. Charge corresponding to each pixel color is separately stored on and read out from the shared floating diffusion node. However, such conventional arrangements in which multiple pixels of different colors in a Bayer mosaic pattern share a common floating diffusion node are not ideal for floating diffusion node summing or charge binning. For example, some conventional arrangements couple four pixels in the Bayer mosaic array to a common floating diffusion node (i.e., four pixels arranged in a common column or four pixels arranged in a unit cell of the Bayer pattern). In such an arrangement, however, the number of pixels of a common color plane that can share a common floating diffusion node is limited, and only a partial summing of charges can be obtained on the array. In some scenarios in which binning of charges from additional pixels is desired, summing operations are performed off-array using the analog or digital periphery logic. However, off-array summing operations increase power consumption and potentially add noise to the summed signals. In other scenarios, more adjacent pixels of different colors in the array are configured to share the common floating diffusion node in order to increase the number of pixels for which charges can be summed. However, arrangements in which more pixels (i.e., nine adjacent pixels of different colors) are coupled to the same floating diffusion node exhibit low conversion gain, decreased voltage sensitivity due to decreased voltage swing at the floating diffusion node, and increased floating diffusion node noise.
In other imaging systems that include pixel arrays with unit cells of two-by-two adjacent pixels of the same color, the adjacent pixels share a common floating diffusion node on which charge is stored prior to read-out. Charge for each unit cell of four pixels of the same color is summed at and read out from the shared floating diffusion node. Because such imaging systems do not use a Bayer mosaic pattern, however, it can be difficult to generate an image having the full-resolution characteristics associated with images generated by sensors that do use the Bayer mosaic pattern.
It would therefore be desirable to be able to provide imaging devices with improved charge-summing capabilities.